Sparse district heating and flexible district heating pipes

Doctoral thesis, 2010

The aim of this thesis is to describe sparse district heating conditions and explore how to improve them. Few sparse areas are heated by district heat, which reflects the economic competitiveness. A method that could be used by the district heating companies to identify the boundaries of profitable district heat is presented. Important key figures are identified when investigating the boundaries of competitive district heat. This method was identified using data gained from research on sparse areas in Göteborg, Sweden and demonstrates what to focus on in order to improve competitiveness. It is concluded that distribution heat losses must decrease in future connected sparse areas to be an efficient and competitive heating alternative. The insulation properties of flexible district heating pipes, widely used when connecting sparse areas, are an important issue when improving the energy efficiency. The long term insulation properties of flexible district heating pipes are experimentally investigated by studying the cell gas transport in semi-flexible polyurethane foams. The results show that the insulation properties are impaired more rapidly than for traditional straight pipes. The flexibility of the material is partly achieved at the cost of faster gas transport. An effective diffusion barrier hindering this gas transport is thus important. A new method to determine the temperature-dependent thermal conductivity of flexible district heating pipes is presented. The method is based on the fact that the temperature decline of hot water in a district heating pipe placed in cool water depends on the thermal conductivity of the polyurethane foam. The temperature decline of the water inside the service pipe is measured and modelled numerically. The difference between measured and calculated temperatures is used to determine the temperature-dependent thermal conductivity and the specific heat by fitting. The method is based on simple measurements of the temperature decline of the service pipe water, and it is applied on single and twin pipes. A single experiment gives the thermal conductivity for a large temperature span.